1. Field of the Invention
The present invention relates to a cellular mobile telephone system and, particularly, to a power control for a closed loop transmitter power control in a code division multiple access (CDMA) cellular mobile telephone system. In more detail, the present invention relates to a technique for carrier synchronization in a coherent detection data communication system.
2. Description of Related Art
CDMA is a modulation and multiple access technique based on spread spectrum communication that has been used to improve the capacity in digital cellular radio communication systems. Code division multiple access using a direct sequence (for convenience, herein called CDMA) is one technique which faciliates communication in systems where a large number of system users are present. In the CDMA system, all the users employ the same code for encoding and decoding of their respective information sequence. Transmitted signals in the common spectrum are distinguished from one another by superimposing a different pseudo-random pattern, also called a code, on each transmitted signal. Thus, a particular receiver can recover the transmitted information intended for it by knowing the pseudo-random pattern, i.e., the key, used by a corresponding transmitter.
The cellular mobile channel typically can be characterised by two separate phenomena, average path loss and fading. The average path loss can be described statistically by a log normal distribution whose mean is proportional to the inverse fourth power of the path distance. The second phenomena has a Rayleigh fading characteristic. The Rayleigh fading process is caused by the physical environment and results in copies of the signal arriving simultaneously from many directions with different transmission delays. This causes significant phase differences between the paths with the possibility for destructive summation, resulting in deep fades. Fading is very disruptive to the channel, and results in poor communications. While Rayleigh fading may be independent for forward (cell-to-mobile) and reverse (mobile-to-cell) links, log-normal shading normally will exhibit reciprocity.
Rapid fading is a basic problem in digital mobile communications. With respect to electric power efficiency, coherent detection schemes are superior compared with the differentially coherent or noncoherent detection. However, carrier recovery, which is necessary for coherent detection, suffers from the time-variant nature of fading channels. The power efficiency obtained by coherency in digital communication systems can be improved only when a carrier synchronization unit is provided in the receiver. Due to implementation considerations and lack of a robust phase estimation algorithm, differential detection or other non-coherent techniques have historically been used in fading channels.
Significant performance improvements can be achieved if near coherent demodulation is realized. A linear modulation scheme such as multi-phase shift keying (M-PSK) or multi-quadrature amplitude modulation (M-QAM) employ coherent reception potentially from highly favorable communications schemes. The power advantage of coherent detection over non-coherent detection remains or is actually enhanced when channel coding or co-channel interference are considered. When the channel is corrupted by Rayleigh fading, resulting in a rapidly varying channel phase, an efficient carrier synchronization unit which derives a carrier signal from the received signal is essential for successful detection. The efficient carrier synchronization is particularly important in a CDMA system for reverse link where a common pilot channel could not be afforded.
Power control is a very important system requirement for a CDMA system, since only by controlling the power of each user can, resources be shared equally among users and the capacity maximized. In order to maximize the capacity of CDMA system in terms of the number of simultaneous calls in a given system bandwidth, the transmitted power of each mobile unit is controlled so that its signal arrives at the cell site with the minimum required signal-to-interference ratio. Power control is essential in any CDMA system in order to mitigate the "near/far problem" and thereby preventing users that are geometrically closer to the base station from "overpowering" users that are farther away. Furthermore, the nature of fading channels causes power variation that must be compensated if it is possible. To equalize the received powers, a combination of an open and closed loop is used. The goal of the open loop is to adjust the transmitted power according to changes in received power. For a reverse link open loop, the mobile stations measure the received power level from the cell sites and adjust their transmitter power in an indirectly proportional manner, in an attempt to have all mobile station transmitted signals arrive at the cell site with the same nominal power level. The open loop control can cope with the very slow shadow type fading.
In the power control in the reverse link, the base station measures the related receiving power level or more precisely measures Eb/Io (ratio of signal energy per bit Eb, to interference power spectral density lo) of the respective related mobile stations and compares the measured ratios with an adjustable threshold. A determination is made to transmit a power-up command or a power-down command to the mobile station. The power adjustment command signals the mobile station to nominally increase or to decrease the mobile station transmit power by a predetermined amount. The rate of power adjustment command transmission must be high enough to permit tracking of slow Rayleigh fading, approximately 1000 commands per second. The power adjustment command is sent to the mobile station in the forward channel addressed to the mobile station. The mobile station combines the received adjustment commands with an open loop estimate to obtain the final value for the transmitted radiated power.
The goal of the closed loop is to provide rapid corrections to the open loop estimate in order to maintain the optimal transmit power. This close loop correction accomodates gain tolerance and unequal propagation losses between the forward and reverse links. The variations in relative path losses and shadowing effects will generally be slow enough to be controlled. The slow Rayleigh fading could be controlled too. However, the variation associated with fast Rayleigh fading could be too rapid to be tracked by power control. It is known that effectiveness of the combination of interleaving and coding in combating the effects of power variation due to slow Rayleigh fading is reduced. At low speed (slow fading) the power control reacts to compensate for fading. The power control and interleaving/coding are most effective in complementary parameter regions, thus providing a degree of robustness for both fast and slow Rayleigh fading. Another benefit of power control is that each user transmits only as much energy as is required, thus prolonging battery life in portable transmitters.
The bit-error-rate/frame-error-rate(BER/FER) performances in a CDMA system are directly related to the closed loop power control efficiency in combating the effects of received power variation. Further, by controlling the power, unnecessary system interference is minimized, increasing overall system capacity. The accuracy of Eb/Io measurement for the purpose of closed loop power control is essential for CDMA cellular system performance, so that a receiver could overcome deleterious fading, providing a required degree of robustness. The Eb/Io measurements for low signal to interference ratios suffer high degradation and introduce errors in power control. For precise power control, accurate and reliable Eb/Io measurements are required.
In a coherent detection data communication system for the reverse link, the known pilot symbols are usually inserted and periodically transmitted with data symbols. The receiver interpolates the channel measurement provided by the pilot symbols to obtain a phase and amplitude reference for coherent detection. The transfer function of the channel is estimated by using the pilot symbols and the data symbols are detected on the basis of the estimated transfer function. However, ordinary symbols interpolations like linear interpolation, low-pass filter interpolation and Gaussian interpolation, have redundancy and could not accurately estimate fading multiplicative distortion without a pilot symbol rate increase. The same pilot symbols are used for Eb/Io measurements. In order to minimize losses caused by the transmission of the pilot symbols, the ratio of the transmitted pilot symbols to the transmitted data symbols is usually low, and using only pilot symbols for Eb/Io measurements could not always satisfy requirements for accurate Eb/Io measurement.
When the number of pilot symbols per slot is small, it is impossible to precisely estimate signal power (S) and interference power (I) for the purpose of closed-loop power control by using only pilot symbols. The usage of data decisions and carrier estimates obtained by an ordinary interpolation for power measurements a significant improvements compared with the case where only the pilot symbols are used. According to this method, the re-modulated signal, that is, the interpolated reference, is calculated by using the decisions and estimated carriers. Signal power is calculated by using the re-modulated signal. The interference power is calculated by using difference between the received signal and the re-modulated signal. However, the usual estimation of the carrier using the usual ordinary pilot interpolation suffers a degradation for low signal power to interference power ratio (S/I) and further improvement in Eb/Io measurement is possible if the carrier signal could be estimated more accurately.
Further, when the number of pilot symbols per slot or frame is small and the S/I ratio is low, particularly for a high diversity order when S/I ratio per path is low, the carrier estimation error of an ordinary pilot interpolation becomes large and the degradation in carrier estimation causes a degradation in receiver BER performance.